Cook noodles until tender in boiling salted water.,Meanwhile, sautee onions, garlic, and mushrooms.,Add the chicken and sautee until just cooked through.,Season with the tarragon, salt, and pepper.,Remove this mixture from the pan and place in casserole dish.,Sauce: Deglaze pan with white wine.,Add chicken stock and bring to a simmer.,Add cream cheese and sour cream and stir to combine.,Thicken sauce with flour (mix with water first to avoid lumps).,Season with salt and pepper to taste.,Pour over chicken & veggies in casserole dish and mix inches.,______,Topping: Crush cornflakes, and mix with parmesan, garlic salt, and oil.,Spread on top of casserole.,Bake at 350F for 20 minutes or until topping is golden.,Serve with a fresh garden salad.